1. Field of the Invention
This invention relates to attitude control for pitch momentum biased satellites and more particularly to control of the roll and yaw errors introduced by orbit inclination deviations from the nominal orbit plane.
2. Description of the Prior Art
Earth satellites now in use for the relaying of telecommunications between widely separated earth stations are placed in circular equatorial orbits at an altitude such that the period of revolution is equal to the period of rotation of the earth. Thus, in principle, the satellite remains at a fixed point in the sky as seen by an observer on the surface of the earth. It is thus ideally geo-stationary. It may also be said to be in a synchronous equatorial orbit. The satellite is, however, subject to the perturbing effects of the sun and moon and of the oblate earth. These perturbing effects manifest themselves in changes in the shape or orientation of the satellite orbit and also in the changes in the position of the satellite along the orbit. The slow drift of the satellite along its orbit (east-west drift) can be due to deviations from the nominal radius of the orbit and can be corrected by relatively small expenditures of propellant fuel by thrusters on board the satellite. Changes in the orientation of the satellite orbit (north-south drift) results in a satellite orbit which deviates from the nominal orbit inclination. As a result of this deviation, roll and yaw pointing errors are introduced. The inclination of the orbit changes with time and can be maintained near zero by the expenditure of considerably more propellant fuel than is required to compenate for east-west drift. For a satellite lifetime of a number of years, the mass of the propellant required for north-south station keeping can become a very substantial fraction of the total mass of the spacecraft. An extended analysis of the perturbation of the orbit of earth satellites has been described in a publication by the Rand Corporation Santa Monica, California by R. H. Frick, entitled "Orbital Regression of Synchronous Satellites Due to the Combined Gravitational Effects of the Sun, the Moon, and the Oblate Earth." (Report R-454-NASA, August, 1967). Accordingly, a system for directly compensating for the roll and yaw pointing errors introduced by the deviations from the nominal orbit inclination is needed in order to avoid the necessity of requiring the spacecraft to carry a large dedicated mass of the propellant for this particular purpose.